Lake Toplitz (German: Toplitzsee) is a lake situated in a dense mountain forest high up in the Austrian Alps, 98 km (61 mi) from Salzburg in western Austria. It is surrounded by cliffs and forests in the Salzkammergut lake district, within the Totes Gebirge (dead mountains). The Toplitzsee water contains no oxygen below a depth of 20m. Fish can survive only in the top 18m, as the water below 20m is salty, although bacteria and worms have been found below 20m that can live without oxygen.

During 1943-44, the shore of Lake Toplitz served as a Nazi naval testing station. Using copper diaphragms, scientists experimented with different explosives, detonating up to 4,000 kg charges at various depths. They also fired torpedoes from a launching pad in the lake into the Tote Mountains, producing vast holes in the canyon walls. Millions of counterfeit pound sterling notes (£100+ million) were dumped in the lake after Operation Bernhard, which was never fully put into action. There is speculation that there might be other valuables to be recovered from the bottom of the Toplitzsee. The speculation is due to there being a layer of sunken logs floating half way to the bottom of the lake, making diving beyond it hazardous or impossible. Gerhard Zauner, one of the divers on the 1959 expedition reports that he saw a sunken aircraft below this layer.[1]

The area is accessible only on foot by a mile-long path, as the K-Mautner-Weg is a private road that serves the Fisherman's Hut restaurant at the western end.

Lake Toplitz is mentioned in the scene in the James Bond movie Goldfinger, where Bond receives the gold bar used to tempt Auric Goldfinger; the bar is said to have been part of a Nazi hoard that was recovered from the lake.

Also in the TV series Private Schulz Lake Toplitz (1981) serves as location where 50 million forged British pounds are being dumped by the Nazis. Filming for the scene in question however took place at a reservoir in South Wales.

The novel The Salzburg Connection by Helen MacInnes involves Nazi gold found in the lake.